1. Field of the Invention
The present invention is directed to an apparatus for treating atrial fibrillation, and in particular to an apparatus for doing so by means of multi-site pacing.
2. Description of the Prior Art
A healthy heart functions to pump blood through the circulatory system in successive, periodic cycles each including an atrial contraction followed shortly thereafter by a ventricular contraction. The successive atrial and ventricular contractions occur upon being triggered by the heart's natural pacemaker, which causes electrical wavefronts to propagate through cardiac tissue, causing the tissue cells to be momentarily polarized, thereby causing the contractions. If a patient's natural pacemaker, through disease, ceases to function or functions only erratically, artificial pacing therapy can be provided by an implanted pacemaker, which delivers low-energy pacing pulses to the atrium, or to the ventricle, or to both the atrium and the ventricle in a properly synchronized sequence. Depending on the needs of a particular patient, the pacemaker can be operated to continuously supply such pacing pulses without interruption, or can operate to sense when the patient's natural pacemaker has failed to deliver a signal resulting in contraction, and only then does the implanted pacemaker deliver a pacing pulse. Pacemakers of this latter type are known as demand pacemakers.
Fibrillation, in general, characterizes abnormal operation of the heart, which can spontaneously occur, wherein the normal propagation of the electrical wavefronts becomes chaotic and therefore the cardiac tissue never receives a clear or coherent signal triggering contraction, and pumping therefore ceases. Ventricular fibrillation is a life-threatening condition, and when it occurs must be treated rapidly and effectively. For this purpose, implantable defibrillators are well-known in the art, which deliver one or more high-energy electrical pulses to the cardiac tissue, at selected locations and in a selected timing sequence, so as to momentarily depolarize substantially all of the cardiac tissue, thereby rendering virtually all of the cardiac tissue momentarily unable to propagate the chaotic wavefronts. If defibrillation is successful, when the cells again become capable of propagating a pacing wavefront, they will do so in a normal, non-chaotic manner.
Atrial fibrillation is usually not a life-threatening pathology, and can be tolerated for a certain amount of time without significant adverse consequences to the patient. This means that upon the occurrence of atrial fibrillation, there is usually a relatively long time during which an effective therapy can be developed, and subsequently administered. Although implantable defibrillator technology, primarily intended for treating ventricular fibrillation, can be adapted also to treat atrial fibrillation, the delivery of high energy shocks to the patient is painful and moreover, such drastic therapy is usually not necessary in the case of atrial fibrillation. Atrial fibrillation is also treated by extracorporeal delivery of the shocks to the heart through the skin of the patient by an external defibrillator of the type well-known in the art, also being extremely uncomfortable for the patient. Moreover, this type of treatment generally results only in temporary relief for patients, and must be repeated.
In treating atrial fibrillation by means of electrical shocks supplied to the heart, such shocks must be applied in synchronism with the ventricular electrical activity otherwise ventricular fibrillation may be induced.
Another treatment regimen for atrial fibrillation is the administration of suitable drugs for reducing the occurrences of atrial fibrillation. Drugs suitable for this purpose which are currently available, however, have many undesirable side effects, and many patients become resistant to their atrial fibrillation suppressing properties, thereby significantly reducing the therapeutic effect of such drugs.
Another type of cardiac arrhythmia is tachycardia, which is a condition whereby the heart's natural pacemaker begins to cause contractions at an abnormally rapid rate. In many instances, tachycardia can be treated by administering pulses at the same energy content as normal pacing pulses, but the anti-tachycardia pulses are delivered in a particular sequence which is designed to return the heart to its normal pacing rate. A conventional cardiac pacemaker, however, has only one pacing electrode disposed in electrical contact with atrial tissue and/or one pacing electrode disposed in electrical contact with ventricular tissue. Administration of pulses having an energy comparable to pacing pulse energy, at a single site in the atrium, has never been shown to terminate atrial fibrillation.
The reason why single site pacing has not been successful in terminating atrial fibrillation is that atrial fibrillation is characterized by the existence of several propagation wavefronts. It has been shown that pacing at one site in the atrium will influence wavefront loops only within a region having a diameter of approximately three centimeters around the pacing site. Since the other propagation loops are thus unaffected by pulses applied to this single site, it has not been possible to terminate atrial fibrillation by supplying such pulses only to one site. Many articles have been published regarding ventricular fibrillation which state that a condition for successful termination of ventricular fibrillation is to ensure that no wavefront loop, or only one wavefront loop, remains after administration of the defibrillation therapy. Atrial fibrillation is similar to ventricular fibrillation, and the same conditions should apply for the successful termination of atrial fibrillation.